I don't know. People do not take cameras with them anymore and the SE 2020's camera system is much better than that of the 6s. So, if you have the money, I think that is worth the new SE alone.
Apart from that, I agree that the 6s is still surprisingly smooth and doesn't seen to struggle with recent iOS versions and applications, despite being a 5 year old phone.
That's true because Apple puts significant effort into making that work. I personally got bugs assigned to me when certain animations were <60fps on the 6s, even if they worked on newer phones like the xs. The 6s is especially prone to this as i recall because the point:pixel scaling is 3x vs 2x/4x on other phones.
Btw - thank you for working hard at making this smooth. My SO had a 6S until 2 months ago and only during iOS 11 it was not smooth. iOS 12 and 13 were perfect and apart from the camera she says she kind of misses the 6S for its smaller form factor. The other benefits of newer generations simply don't matter to many users. WhatsApp/Insta/Email/Maps runs just fine on an A9 or A13 and her main game is 2D chess....
Apart from that, I agree that the 6s is still surprisingly smooth and doesn't seen to struggle with recent iOS versions and applications, despite being a 5 year old phone.